The present invention relates in general to optical transmission fibres, and more specifically, to the reduction of dispersion effects in optical transmission fibre systems.
A communications system may employ an optical transmission fibre to transmit digital or analogue information. In such case, the information is typically sent along the fibre as light pulses. In order to accommodate several different channels on one fibre, the light pulses for each channel have a different nominal frequency (or wavelength). However, a train of optical pulses associated with a single channel is not in fact composed of a single optical frequency but a spectrum of frequencies extending over a frequency band. The bandwidth associated with these optical frequencies of a channel is usually directly related to the data rate associated with that channel: where channels have high data rates (e.g., 160 Gb/s), the bandwidth is large (requiring a sufficient spacing between channels to avoid overlap). Different wavelengths of light propagate along an optical transmission fibre at different speeds: this property is known as chromatic dispersion (CD). CD measurements characterise how the velocity of propagation in an optical fibre or other optical component changes with wavelength. Thus, conventionally, dispersion is measured as units of picoseconds per nanometer per kilometer (ps/nm/km). If an optical pulse has a large bandwidth (i.e., it is composed of a large number of optical frequencies) the CD causes the pulse to change its temporal profile. The change in temporal profile associated with the CD may result in reduced system performance limiting the distance that the information may be propagated without electronic regeneration. For this reason it can be important to control the CD of the optical system for the wavelengths associated with a single optical channel.
The properties of the optical fibre often result in CD being wavelength dependant. This rate of change of CD as a function of wavelength is commonly called dispersion slope. In Dense Wavelength Division Multiplexed (DWDM) systems employing many different optical channels not only must the CD be managed but also the dispersion slope must be compensated. For optimal performance the total CD (for the whole optical system) of all wavelengths propagated down a single optical fibre must be maintained at a constant value (not necessarily 0 ps/nm/km). Failure to do so results in the temporal profile of optical pulses in some channels changing due to dispersive effects as previously explained. Dispersion slope is a particular problem for optical channels in the commonly used C band (1.530 xcexcm to 1.562 xcexcm) and L band (1.570 xcexcm to 1.602 xcexcm) of the Erbium Doped Fibre Amplifier (EDFA). The dispersion characteristics of an optical fibre are represented by the relative dispersion slope (RDS) of the fibre. The RDS at a given wavelength is defined as the dispersion slope at that wavelength divided by the absolute value of the dispersion at that wavelength.
Optical fibres used in the transmission of optical signals typically have positive dispersion over the wavelengths used by the channelsxe2x80x94such that (absent xe2x80x9cchirpxe2x80x9d, a factor discussed hereinafter) the optical frequencies associated with a pulse spread apart as they travel on the fibre. Further, typical transmission fibres have a positive dispersion slopexe2x80x94meaning that the magnitude of the dispersion on the fibre is greater for light of a longer wavelengths than it is for light of shorter wavelengths. To control dispersion effects on such fibres, dispersion compensating fibre (DCF) is used; DCF has a negative dispersion over the wavelengths used by the channels and a negative dispersion slope, the value of which may be varied to some extent by appropriate design and manufacture of the fibre. The use of DCF in optical links having a transmission fibre characterised by positive dispersion and a moderate positive RDS throughout the transmission wavelengths (such as non-dispersion shifted fibre (NDSF)xe2x80x94also known as standard fibre or single mode fibre (SMF)) has generally provided satisfactory slope compensation. However, the use of DCF in optical links employing positive dispersion fibre with a large positive RDS at the transmission wavelengths (such as Large Effective Area Fibre (LEAF(trademark)) manufactured by Corning Inc. and TrueWave(trademark) Reduced Slope Fibre (TWRS) manufactured by Lucent Inc.) has provided poor slope compensation. A large positive RDS means that the dispersion slope at a wavelength is large as compared with the actual dispersion at the wavelength. Thus, there is a large percent change in the dispersion as the wavelength increases and it has proven difficult to compensate large percent changes in dispersion with the use of DCF.
The result is that when commercially available DCF is used to CD compensate some transmission fibres, the CD experienced by different channels in the DWDM system is not substantially the same and therefore not all channels experience the optimum CD. This can result in poor performance and high Bit Error Ratio (BER) for some optical channels. This in turn limits the total capacity or reach of the optical system.
Another approach to compensate for dispersion is to introduce a dispersion compensation system for each channel (frequency) of an optical transmission system. However, this approach is expensive.
Therefore, there is a need for a cost effective manner of more fully compensating for the effects of dispersion of optical signals on communication links.
With the present invention, the effects of dispersion of light on a link having an optical transmission fibre characterised by positive dispersion and large positive relative dispersion slope (xe2x80x9cRDSxe2x80x9d) can be reduced. To do so, a sufficient length of dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d) is employed so that dispersion slope on the link is substantially nulled, which will normally result in the net dispersion of the link being made negative. To compensate for the net negative dispersion, optical signals may be launched on the link with a positive chirp.
According to an aspect of the present invention, there is provided a method for reducing effects of dispersion of optical signals on a communication link having an optical transmission fibre with positive dispersion and a large positive relative dispersion slope, comprising: passing said optical signals through a sufficient length of dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d) having a negative dispersion and negative dispersion slope so that dispersion slope of said link is substantially nulled; where, in consequence of said length of DCF, net dispersion on said link is negative, launching said optical signals on said link with a positive chirp.
According to another aspect of the present invention, there is provided a method for transmitting optical signals comprising: passing said optical signals through an optical transmission fibre with a positive dispersion and a large positive relative dispersion slope; and passing said optical signals through a sufficient length of dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d) having a negative dispersion and a negative dispersion slope so that dispersion slope on said link is substantially nulled and net dispersion on said link is negative.
According to a further aspect of the invention, there is provided a communication link having reduced dispersion effects comprising: an optical transmission fibre with a positive dispersion and a large positive relative dispersion slope; and at least one dispersion slope compensation module (xe2x80x9cDSCMxe2x80x9d) operatively coupled to said optical transmission fibre, each said DSCM having a dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d), said at least one DSCM providing DCF of a length sufficient that dispersion slope on said link is substantially nulled and net dispersion on said link is negative.
According to another aspect of the invention, there is provided a method of designing a communication link having an optical transmission fibre with a positive dispersion and large positive relative dispersion slope comprising: choosing a length of dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d) of sufficient length so that dispersion slope on said link is substantially nulled and net dispersion on said link is negative; and choosing at least one positive chirp transmitter for launching optical signals on said link.
According to a further aspect of the invention, there is provided a kit for a communication link having an optical transmission fibre with a positive dispersion and large positive relative dispersion slope, comprising: at least one dispersion slope compensation module (xe2x80x9cDSCMxe2x80x9d) having a dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d) such that said at least one DSCM provides a length of DCF sufficient so that, once said at least one DSCM is installed on said link, dispersion slope on said link is substantially nulled and net dispersion on said link is negative; and at least one positive chirp transmitter for launching optical signals on said link.
According to a yet further aspect of the invention, there is provided a communication link comprising: at least one length of optical transmission fibre having a positive dispersion and a large positive relative dispersion slope for carrying a plurality of frequency multiplexed optical channels; at least one dispersion slope compensation module (xe2x80x9cDSCMxe2x80x9d) having a length of dispersion compensating fibre (xe2x80x9cDCFxe2x80x9d) so that net dispersion slope along said link is substantially nulled and net dispersion of said link for said optical channels is negative; at least one positive chirp transmitter, said at least one positive chirp transmitter for launching each said channel.
Other aspects and features of the present invention will become apparent to those skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.